In any farming activity, culture site selection is of paramount
importance. Technological and economic considerations play a
major role in the selection process. A careful appraisal of the
habits of the organism to be cultured would give a resonable
level of confidence on the tolerance limits within which the
various environmental parameters can vary. Due consideration has
to be given to possible effects of fluctuating water flow,
primary production, siltation, etc. in order to obtain the
optimum level of growth and production of high quality pearls.
Unsuitable levels of environmental factors such as salinity,
water temperature, cold water currents and other factors such as
red tides, hydrogen sulphide and pollution by industrial and
domestic effluents are serious hazards to pearl culture.

Sheltered bays are ideal locations for pearl oyster farms. They
offer good protection to the culture structures such as rafts and
cages. Shallow coastal waters where the sea is calm most of the
year can also be considered as a suitable site.

In temperate regions, the water temperature plays an
important role in the biological activities of pearl
oysters. In Japan, the optimum temperature for oyster
growth has been found to be between 20–25 °C. A
temperature below 13 °C causes hybernation. Below 6 °C,
the oysters die. At temperatures above 28 °C, the oysters
show exhaustion. The thickness of the pearl layers are
affected by the minute changes in water temperature during
the day and also vary considerably according to the season
of the year. The deposition of calcium stops at a water
temperature of 13 °C. In the Gulf of Kutch, the oysters
grow vigorously in winter months when the seawater
temperature ranges between 23–27 °C. A slight decrease in
temperature triggers spawning in oysters in the Gulf of
Mannar. The growth-temperature relationship is presumably
valid only up to a certain temperature for optimum growth.

Pearl oysters tolerate a wide range of salinity from 24–50
‰ for a short duration of 2–3 days. Salinities of 14
‰ and 55 ‰ may cause a 100 % mortality among the
oysters. The effect of salinity on the growth of pearl
oyster has not been clearly investigated. However, it
appears that pearl oysters tend to prefer high salinities.
Oysters raised in such salinities produce pearls with a
golden tint.

Gravelly bottoms are suitable for pearl oyster farming,
while sandy or muddy bottoms should be avoided. Oyster
growth is affected by water temperature and nutritional
condition of the ground. Repeated culture on the same
ground leads to some extent the deterioration of pearl
quality. The chemical and physical state of the sea bottom
is affected by the organic substances discharged from the
oysters and fouling organisms. Periodic removal of such
accumulated substances from the bottom of the culture
grounds often increase production as well as quality.

Pearl oysters generally prefer clear waters as high
turbidity levels will affect their filtration efficiency.
A decline in oyster condition was noted at Veppalodai farm
due to the high silt content in the farm area throughout
most of the year.

Culture sites should be naturally sheltered against strong
winds and waves. Tidal amplitude and currents must be
sufficient in order to allow replenishment of oxygenated
water and fresh plankton and flush away waste materials.
In strong water currents the formation of the pearl layers
is usually fast, but the quality of pearls produced is
affected.

The condition of a specific culture ground depends
primarily on the chemical constitution of the seawater and
on the species and amount of plankton present. Rich
nutrients discharged by rivers into the sea are responsible
for high primary productivity. The oysters probably derive
their chief source of conchiolin from the nitrogen
substance of the plankton. The organic matter and calcium
dissolved in the seawater are directly absorbed by the food
consumption cells. The calcium passes through the mantle
to be deposited on the surface of the shell or pearl in the
process of their formation. The presence of trace metals
in small quantities influences the colour of the nacre.

In pearl oyster farming, oysters collected from the natural beds
or reared from naturally collected or cultured spat are used.
In the Gulf of Mannar, several pearl banks are distributed off
Tuticorn at a distance of 12–15 km and at depths of 12–25 m.
Pearl oysters from these beds are collected by skin and SCUBA
diving (Plate IV A). Wide fluctuations in terms of pearl oysters
availability have been noted in different pearl banks in this
area during the last few centuries as also during the most recent
years. In the Gulf of Kutch, the pearl oysters are found on the
intertidal flats and the population is sparse. Collection is
done by hand.

In Japan, oyster spat are collected by submerging bundles of
cedar twigs near the water surface during the peak larval
settlement season. Hyzez films and old fish nets are also
commonly used as spat collectors. Almost the entire requirement
of oyster supply to the pearl culture industry is met by this
type of spat collection. Spat collection attempts in India have
not been successful, and this may be due to the distance of the
pearl oyster beds from coastal waters.

However, India has recently succeeded in producing pearl oyster
seed under hatchery conditions, therby providing the industry
with a more dependable source of oysters.

Raft culture is considered to be one of the most suitable
farming methods in sheltered bays. The size of the rafts
can be altered according to the convenience of the
situation. A raft of 6×5 m in size can be easily
constructed and floated with 4 buoys. Rafts are usually
constructed with logs of teak, venteak or casuarina wood,
of chosen length with the bottom of about 10 cm diameter
tapering to 6 cm diameter at the tip. These logs are
arranged as per the requirement and lashed with coir ropes.
Floats are attached to the raft to give buoyancy. The
floats can be sealed empty diesel drums of 200 l capacity
with fibreglass coating, mild steel barrels painted with
antisaline/anticorrosive paints or FRP styrofoam floats
(Plate IV B and C). Unit raft system is found to be
convenient and well suited to the Indian sea conditions.
Rafts are moored with anchors at opposite sides with tested
quality chains and their direction is decided according to
the prevalent wind direction at the specific site (Fig. 4
A-C).

In the long-line culture method, spherical or cylindrical
floats which are connected by horizontal synthetic rope or
chain are used (Plate IV D; Fig. 4 D). The oyster cages
are suspended from the ropes. This system is good for open
sea conditions. In another method of hanging, a hole is
drilled near the hinge of the pearl oyster. A small thread
is put through the hole, which is then tied to a straw rope
coated with tar. The straw ropes are hung from a raft.

Sea bottoms with a granite or coral stones composition can
be used for on-bottom culture. In the Tuticorin Harbour
Basin where the breakwater has been constructed with
granite stones, the protected portion of the breakwater is
used for culturing mother oysters. 1 m of water is
available below the low water mark. Due to constant
circulation of seawater, settlement of fouling organisms is
poor and inconsistent. However, it has been noted that the
growth of the mother oyster is slower in on-bottom culture
compared to the growth of oysters cultured in raft.

Box cages, measuring 40×40×15 cm, are used to rear mother
pearl oysters. The size of the mesh varies with the size
of the oysters to be reared. The frames of the cages are
made up of 6 mm mild steel rods, coated with anticorrosive
paints or coal tar. Box-cages are useful in general mother
oyster culture (Plate V A).

To trace the history and performance of individual oysters,
frame nets are used. The frames, measuring 60×40 cm each
with five compartments, meshed and hinged at one end, open
as a book. The oysters are arranged in rows and held in
the compartments when closed. The space available in
between the two frames is about 10 mm which is sufficient
for the oysters to open their valves for feeding and
respiration (Plate V B).

Juvenile pearl oysters are reared in netcages (Plate V C-D).
Synthetic fabric of velon screen bags whose sides are
stretched with a steel rod in the form of a prism are used
for rearing of juveniles. The mesh size of the screen
depends on the size of juveniles to be reared. The mouth
of the bag is tied with a synthetic twine which facilitates
opening or closing when required. To provide further
protection from predators the bags are placed in old nylon
fish net bags. Clogging by silt and by the growth of
fouling organisms can be prevented by periodical
replacement of the velon screen bag which can be cleaned,
sun-dried and reused. Spat of up to 2 cm in size are
reared in these small netcages. Box-cages which are used
for rearing mother oysters can also be used for juvenile
rearing by providing an additional velon screen cover
inside the cage.